Adhesives such as pressure sensitive adhesives can be prepared using a variety of polymerization methods, including solution, emulsion, and bulk polymerization. An example of a bulk polymerization process is a free radical photopolymerization process in which ultraviolet radiation is used to initiate polymerization.
One problem with traditional on-web polymerization methods for preparing adhesives is that it is difficult to prepare relatively thick adhesives at a rate suitable for commercial production. In the case of solution and emulsion polymerizations, the low viscosity of the polymerizable composition limits the thickness of any one layer that can be prepared, requiring multiple coating, polymerizing, and drying steps to build thick adhesives layer by layer.
In free radical ultraviolet radiation-initiated processes, the polymerization rate is a function of the illumination intensity and the fraction of light absorbed by the photoinitiator at each wavelength. Typical ultraviolet light sources used for polymerization emit broad band radiation with relatively low intensity at any one wavelength.
To compensate for the low intensity and achieve a reasonable polymerization rate, it is necessary to increase the fraction of light absorbed, either by increasing the initiator concentration or by increasing the absorption coefficient of the initiator at the wavelengths provided by the source. However, increasing the absorbance results in an increased cure gradient throughout the thickness of the coating, with the top layers being cured to a greater extent than the lower layers. This is especially detrimental in the case of relatively thick coatings (e.g., on the order of 2 mils or greater).
It has been suggested to use monochromatic radiation having a peak intensity at 222 nanometers as a light source. Such radiation, however, is not optimal for polymerization of thick acrylic samples because both the unsaturated acrylate monomers and the acrylate polymer absorb at this wavelength, thereby decreasing the amount of light available to initiate polymerization.